This invention relates to an electric propulsion system for a multi-wheeled vehicle including but not limited to a bicycle. Electric powered vehicles are inherently cleaner and quieter than their gasoline and diesel powered counterparts, with no warm-up and potentially less maintenance required.
Electric powered bicycles are well known in the art at least as far back as the Humber electric bicycle of 1898. There have been many variations over the vows including the patents of Rudwic (U.S. Pat. No. 4,280,581), Havener et al (U.S. Pat. No. 3,966,007), and McCulloch et al (U.S. Pat. No. 3,921,745).
Standard components of a typical electric bicycle include a bicycle, a conventional lead-acid battery, a DC electric motor, drive means for mechanically connecting the motor with a front or rear wheel, and a throttle means for electrically connecting the battery with the motor. There is invariably a provision whereby pedal and motor power may be provided in combination to add range between recharges and improve speed and hill climbing ability. There is sometimes a regenerative braking arrangement such that the motor acts as a generator when slowing or stopping to convert the forward momentum of the bicycle and rider into a charging current to restore energy to the battery.
Many difficulties and optimization problems have been noted in prior art. One of the major problems is that a conventional lead-acid battery stores only a small fraction of the energy contained in an equal weight of gasoline or diesel fuel. Although nickel-cadmium and other types of alternate battery offer improved energy density, capacity has not yet approached that of gasoline or diesel fuel and cost may be prohibitive.
A related problem is that an electric propulsion system invariably interferes with the normal pedaling and handling of a bicycle. Increasing the size of the battery from some base size likely increases range between recharges, but further hampers pedaling and handling. Speed and hill climbing ability may also be negatively affected. Decreasing the size of the battery from some base size likely improves pedaling and handling, though decreases range between recharges and may affect speed.
The DC permanent magnet motor has been widely referenced as a powerful, efficient, yet low cost option. One disadvantage is that frequent ratio changes are needed to stay within the 1,500 to 3,000 RPM window of highest power and efficiency of this motor, and simultaneous changing ratios for both motor and pedals is too difficult for most operators. The single ratio used in most prior art compromises either speed or hill-climbing ability to an unacceptable extent, and a simple yet highly efficient automatic transmission is lacking.
An electric motor of about 1/10 horsepower providing a top speed of about 11-14 mph on level ground has been cited as optimum. One reason for this selection is that air resistance against a bicycle and rider increase rapidly above about 12 mph, and efficiency likewise diminishes. Another reason is that by having power parity with the unmotored bicycle (the average rider also generates about 1/10 horsepower with normal pedaling), the electrical bicycle can be used over normal bicycle routes and otherwise function as a conventional bicycle.
Regenerative breaking can extend the range of the electric bicycle between recharges by a significant amount. However, a relatively high motor RPM is generally needed to overcome the terminal voltage of the battery before charging can begin. For example, if a 12 volt permanent magnet motor and a 12-volt battery are used under a ratio suitable for level ground, there would be little charging (or braking) at speeds below about 8 mph, and the variable braking action for a slow or rapid decrease in speed would be lacking. These problems have been partially solved in prior art through a variety of devices, though progress seems to have come at the expense of complexity.
Where a friction clutch is used, an increase in efficiency and a decrease in premature wear is gained if a clutching force proportional to the load applied can be provided. A relatively high amount of friction is needed on first starting, a medium amount when crossing level ground, and a low amount when going down hill, whereas a spring type clutch provides a constant clutching force regardless of conditions.
One clutching solution noted in prior art entails mounting a motor and drive roller on a hinged or slidable frame. As the drive roller swings or slides into contact with the driven tire, the opposing rotation between drive roller and driven tire produces a "gripping" action proportional to load applied. One serious flaw is that only a single ratio between the motor and the driven wheel is realized.
Better handling and increased safety result if the motor can be throttled for a variety of power levels. The throttle means of most prior art, whereby the motor is switched between ON and OFF, is very sub-optimal in this regard. Another throttle means of prior art, which causes a plurality of batteries to be switched into parallel or series combinations for at least two discrete voltage levels, with the lower voltage level also useful in regenerative braking, offers some improvement over direct switching. The rheostat or dropping resistors of other yet other prior art offers additional or even continuous voltage levels, though dissipates an unacceptable amount of power into heat.
The throttle means of still other prior art, which incorporates a DC to DC converter to regulate voltage to the motor through varying the duty cycle of the applied battery voltage, provides a continuously variable voltage level at improved efficiency over a rheostat or dropping resistors. However, full battery voltage is never delivered at full throttle because of the presence of an inline semiconductor, complexity is relatively high, and efficiency remains decidedly below that of direct switching.
In summary, designing a practical electric propulsion system for a bicycle has proven to be a formidable task. As evidenced by the absence or rarity of electric powered bicycles in commercial production at this time, an electric bicycle which effectively competes against other forms of transportation remains lacking.